1. Field of the Invention
The present invention relates to a process for the conversion of C.sub.8 aromatic hydrocarbons and, more particularly, it is concerned with a process for converting or transforming effectively a mixture of C.sub.8 aromatic hydrocarbons containing ethylbenzene and xylenes by the use of an acid leached hydrogen type mordenite catalyst having a particular silica/alumina ratio.
2. Description of the Prior Art
Of late, large amounts of C.sub.8 aromatic hydrocarbons have been obtained by separation from distillates of coal tar, reformed products or thermally cracked products of petroleum, etc. However, the principal demand for these C.sub.8 aromatic hydrocarbons is, in fact, limited to p-xylene and o-xylene and, accordingly, many proposals have hitherto been made on methods of converting the residual C.sub.8 aromatic hydrocarbon mixture, from which such xylene isomers have been recovered, into the useful components by isomerization or disproportionation. For example, it is well known that the isomerization of xylene is carried out in a vapor phase reaction at high temperature around 500.degree. C. using a solid acid catalyst, but this method is not practical since the operation must be carried out at a high temperature and the catalyst deteriorates. In order to solve this problem, there have recently been proposed a method comprising using a silica-alumina catalyst in the liquid phase at a temperature of about 290.degree. to 400.degree. C. under high pressure (see Japanese Patent Publication No. 6460/75 based on Japanese application No. 103397 filed Dec. 24, 1969. ) and a method comprising carrying out the reaction in the liquid phase at a temperature of 0.degree. to 400.degree. C. using a chromium-containing hydrogen type mordenite catalyst (see Japanese Patent Publication No. 88032/75 assigned to Teijin K.K.). However, these methods also have disadvantages. That is, the former method, aimed at preventing deterioration of the catalyst, still requires a rather high temperature and has an insufficient isomerization efficiency, while the latter method requires a troublesome procedure for the preparation of the catalyst because of the addition of chromium. Other references are Japanese Patent Publication No. 3976/74 based on Japanese application No. 24768 filed May 15, 1969 assigned to Universal Oil Products Co.; Japanese Patent Publication Nos. 53335/75 and 53336/75 assigned to Mobil Oil Corp. and Japanese Patent Publication No. 39632/76 based on Japanese application No. 111646 filed Sept. 30, 1974 assigned to Teijin K.K.
U.S. Pat. No. 3,780,123 assigned to Texaco Inc. discloses a disproportionation method with a composite catalyst starting with, e.g., ethylbenzene, in which, however, no xylenes are produced.
On the other hand, a number of methods for removing ethylbenzene present in high concentration in C.sub.8 aromatic hydrocarbons have hitherto been proposed so as to obtain starting materials for the production of p-xylene by isomerization of C.sub.8 aromatic hydrocarbons. One of the prior art methods, therefore, is a superfractionation method, which has many disadvantages; for example, it requires a large installation investment and high operating cost on a commercial scale. A hydrogenation and isomerization method using a noble metal catalyst is also known but this method has the disadvantage that the selectivity of ethylbenzene conversion is not as great as desired, resulting in loss of the xylene isomers present with formation of a large amount of non-aromatic hydrocarbons due to hydrogenation. Furthermore, there has recently been proposed a method for the disproportionation of ethylbenzene in the presence of a chlorine compound using only ethylbenzene as starting material and an acid leached hydrogen type mordenite catalyst with a high silica to alumina molar ratio but this method needs the presence of chlorine which has a corrosive action in the reaction system.